In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications systems frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for terminal devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node at the network side and at the terminal devices at the user side might be required to reach a sufficient link budget.
The terminal devices and/or the transmission and reception point (TRP) of the network node could implement beamforming by means of analog beamforming, digital beamforming, or hybrid beamforming. Each implementation has its advantages and disadvantages. A digital beamforming implementation is the most flexible implementation of the three but also the costliest due to the large number of required radio chains and baseband chains. An analog beamforming implementation is the least flexible but cheaper to manufacture due to a reduced number of radio chains and baseband chains compared to the digital beamforming implementation. A hybrid beamforming implementation is a compromise between the analog and the digital beamforming implementations. As the skilled person understands, depending on cost and performance requirements of different terminal devices, different implementations will be needed.
For terminal devices the incoming signals might arrive from all different directions. Hence it is beneficial to have an antenna implementation at the terminal devices which has the possibility to generate omni-directional-like coverage in addition to the high gain narrow beams. One way to increase the omni-directional coverage at the terminal devices is to install multiple antenna arrays, and point the antenna arrays in mutually different directions. FIG. 1 schematically illustrates a terminal device 100 comprising an antenna arrangement having two antenna arrays 120a, 120b, each comprising antenna elements 140a, 140b and phase shifters 130a, 130b. Each antenna array 120a, 120b is operatively connected to its own baseband (BB) chain 110a, 110b. As the skilled person understands, the terminal device 100 could be provided with further antenna arrays, each having its own baseband chain, and each pointing in its own direction in order to further increase the omni-directional coverage. However, in order to limit the heat generated by the antenna arrangement the number of baseband chains should be kept small, for example by limiting the number of baseband chains to just two baseband chains.
With only two baseband chains, it could be challenging to design an antenna arrangement with high flexibility with respect to coverage and capacity (i.e. an antenna arrangement capable of generating both narrow beams and wide beams pointing different directions).